The invention relates to a fully hydraulic brake booster/master cylinder unit according to the precharacterizing clause of claim 1. Such a brake booster/master cylinder unit is known, for example, from WO 97/18116.
DE 40 29 114 A1discloses a hydraulic brake booster/master cylinder unit with a first booster chamber and a second booster chamber. During operation, hydraulic pressure is fed from the first booster chamber via a pressure-reducing valve, a pressure-regulating valve or a directional or shut-off valve to the second booster chamber, in order to implement specific characteristic curves.
In contrast to the brake booster/master cylinder units still predominantly used today, in fully hydraulic brake booster/master cylinder units the boosting of the actuating force introduced is not achieved with the aid of a vacuum, but purely hydraulically. In a similar way to conventional vacuum brake boosters, the actuating force normally introduced by the vehicle driver via a brake pedal is boosted proportionally, in that hydraulic fluid under pressure is supplied to a booster chamber in a controlled manner by means of a valve, in order to assist the pressure build-up in the pressure chamber or pressure chambers of the master cylinder which are connected to the individual brake circuits of the vehicle brake system. When the brake is released, this booster chamber is then relieved of pressure again in a controlled manner by means of said valve.
It is desirable to design fully hydraulic brake booster/master cylinder units in such a way that the boosting force exerted in each case by the brake booster is variable, so that, for example, adaptation to different load states of the vehicle can take place. In specific driving situations, it may also be desirable to exert an additional braking force, for example when it has been recognized that the actuating force exerted by the driver is not sufficient in order, for example, to avoid a collision with an obstacle. The vehicle driver should, however, notice such actions on the system as little as possible, so as not to be needlessly irritated.